In a voltage regulator circuit the dropout voltage is defined as the input-output voltage differential at which the circuit ceases to regulate against further reductions in input voltage. Common current is that part of the input current to the regulator that is not delivered to the load.
FIG. 1 shows a conventional negative voltage regulator circuit. Terminal 10 is the input terminal to which a source of unregulated negative voltage is applied. The input is referenced to ground which is the common terminal. While this application will show a negative voltage regulator, and the preferred embodiment will be a negative regulator, the circuit and concepts developed will apply equally to positive regulators. Here the same circuits will be employed, but all of the devices would be complemented along with the polarity reversal.
Terminal 11 is the regulated negative output. The circuit controls the conduction in transistor 12 so that the output voltage is at the correct level regardless of input voltage or the output current in a load (not shown) connected between terminal 11 and ground. Driver transistor 13 operates with transistor 12 in a Darlington configuration. Transistor 14 acts as an emitter follower control amplifier that operates the Darlington. Differential amplifier (diff-amp) 15 drives control transistor 14 and has an inverting input connected to a source of constant reference voltage 16. This will typically be a voltage reference of well known construction that develops a potential that does not vary with changes in input voltage or temperature. The noninverting input of diff-amp 15 is coupled to a tap on voltage divider 17 which is coupled between output terminal 11 and ground. In operation diff-amp 15 will drive transistors 14, 13 and 12 to produce a potential at terminal 11 that will result in equality of the potentials into diff-amp 15. Accordingly, the output voltage at terminal 11 is established in part by the ratio of resistors 17a and 17b. Since the Darlington transistor is coupled as an inverter, the feedback loop is negative and of high gain so that the potential at terminal 11 is regulated and very stable.
The voltage regulator of FIG. 1 is very useful, but its dropout voltage is limited to the V.sub.BE of transistor 12 plus the V.sub.SAT of transistor 13. At 300.degree. K. this is typically about a volt. Over temperature it can rise to about 1.2 volts. If lower dropout is desired, transistor 13 can have its collector returned to ground thereby converting the Darlington to an emitter follower driven common emitter transistor. This configuration will display a dropout voltage of V.sub.SAT which is only a small fraction of a volt over temperature. However, in such a configuration, the current flowing in transistor 13 will increase the regulator common current and is therefore undesirable.